This invention relates to an apparatus for the deposition of thin films, and, more particularly, the present invention relates to an apparatus for depositing a multilayered material from a sputtering target.
Memory devices are an extremely important component in electronic systems. The three most important commercial high-density memory technologies are SRAM, DRAM, and FLASH. Each of these memory devices uses an electronic charge to store information and each has its own advantages. SRAM has fast read and write speeds, but it is volatile and requires large cell area. DRAM has high density, but it is also volatile and requires a refresh of the storage capacitor every few milliseconds. This requirement increases the complexity of the control electronics.
FLASH is the major nonvolatile memory device in use today. Typical non-volatile memory devices use charges trapped in a floating oxide layer to store information. Drawbacks to FLASH include high voltage requirements and slow program and erase times. Also, FLASH memory has a poor write endurance of 104-106 cycles before memory failure. In addition, to maintain reasonable data retention, the thickness of the gate oxide has to stay above the threshold that allows electron tunneling, thus restricting FLASH""s scaling trends.
To overcome these shortcomings, magnetic memory devices are being evaluated. One such device is magnetoresistive random access memory (hereinafter referred to as xe2x80x9cMRAMxe2x80x9d). MRAM has the potential to have speed performance similar to DRAM. To be commercially viable, however, MRAM must have comparable memory density to current memory technologies, be scalable for future generations, operate at low voltages, have low power consumption, and have competitive read/write speeds.
MRAM devices are typically fabricated using sputtering deposition systems, such as physical vapor deposition (hereinafter referred to as xe2x80x9cPVDxe2x80x9d) systems or ion beam deposition (hereinafter referred to as xe2x80x9cIBDxe2x80x9d) systems. Such sputter-deposition systems create electromagnetic fields in an evacuated chamber into which an inert, ionizable gas, such as argon, is introduced.
Turn now to FIG. 1 which illustrates a prior art ion beam deposition apparatus 203. Ion beam deposition apparatus 203 includes a vacuum chamber 210. A substrate stage 212 is positioned therein vacuum chamber 210 and a substrate 214 is positioned on substrate stage 212. Substrate 214 can include, for example, a silicon wafer or a similar supporting substrate.
A target holder 250 is positioned within vacuum chamber 210. Target holder 250 is capable of holding at least one holding member, such as a sputtering target. In this example, target holder 250 holds a target 258 with a surface 233, a target 262, a target 294, a target 295, and a target 296, wherein target 258 is initially positioned in a desired sputtering position 259 facing an ion beam source 238. Further, target holder 250 is rotatable about an individual axis 221, as will be discussed presently. Ion beam deposition apparatus 203 typically includes an assist ion beam source 297 to clean substrate 214 and subsequent layers grown thereon.
Ion beam source 238 directs a flux of ions 241 at target holder 250. It is well known by those skilled in the art that when a flux of ions strike a sputtering target, material from the sputtering target is sputtered through a continuous range of angles relative to the sputtering target. Atoms from the ion beam are also scattered from the target into a continuous range of angles. For example, when flux of ions 241 strikes the target in position 259, material from the target in position 259 is substantially sputtered in a direction 222, a direction 234, a direction 216, and a direction 219. Ions and atoms from the beam are also scattered with significant energy into those angles. Further, a stray beam from ion flux 241 is substantially directed in a direction 281. The material generally sputtered in direction 216 will be incident on substrate 214, as desired, to grow a material film thereon. Further, the atoms generally sputtered and scattered in directions 222, 219, and 234 and the stray beam in direction 281 can cause significant contamination within chamber 210 and on substrate 214 by resputtering material from the chamber walls or fixtures. Thus, it is desirable to shield the chamber walls and other regions where contamination may be generated to prevent this resputtered material from reaching substrate 214.
For example, the material sputtered and scattered in direction 219 typically sputters chamber 210 in a region 255 and causes a contamination flux 256. The material sputtered and scattered in direction 219 is generally sputtered at an angle, xcex8xe2x80x2, relative to a reference line 268 oriented parallel to surface 233 of desired sputtering position 259. It will be understood that angle, xcex8xe2x80x2, is typically within a range from 30xc2x0 to 45xc2x0. In the prior art, a baffle 227 is sometimes positioned near region 255 on chamber 210 to shield contamination flux 256. Contamination flux 256 is generally sputtered along a reference line 270 which is not incident to substrate 214 and causes minimal contamination problems.
However, material sputtered in direction 222 typically sputters chamber 210 in a region 224 and causes a contamination flux 226 which is sputtered toward substrate 214 along a reference line 269. The material sputtered and scattered in direction 222 is generally called a forward scattered flux and is sputtered at a shallow angle, xcex8, relative to reference line 268. It will be understood that shallow angle, xcex8, is typically within a range given approximately from 0xc2x0 to 20xc2x0. This forward scattered flux is of particular concern since it typically contains most energetic atoms. Contamination flux 226 is typically sputtered such that baffle 227 is insufficient to shield substrate 214.
The stray beam in direction 281 from ion flux 241 typically sputters chamber 210 in a region 284 and causes a contamination flux 282 which is sputtered toward substrate 214 along a reference line 283. The stray beam in direction 281 is generally a small flux of ions in a tail of the ion beam that misses the target and hits chamber 210 behind the targets in region 284. This stray beam will sputter material from the wall, some of which will deposit with the growing film and result in contamination.
To illustrate a method of operation for apparatus 203, consider the following example. Assume that it is desired to deposit a material layer 211 on substrate 214, a material layer 213 on material layer 211, a material layer 215 on material layer 213, and a material layer 217 on material layer 215 as illustrated in FIG. 1. Further assume that material layer 211 includes material sputtered from target 258, material layer 213 includes material sputtered from target 262, material layer 215 includes material sputtered from target 295, and material layer 217 includes material sputtered from target 296.
Initially, target 258 is positioned in desired sputtering position 259. To deposit material layer 211, ion beam source 238 is turned on. After material layer 211 is deposited, ion beam source 238 is turned off and target 262 is rotated into desired sputtering position 259. Ion source 238 is turned on to deposit material layer 213. After layer 213 is deposited, ion beam source 238 is turned off and target 295 is rotated to desired sputtering position 259.
Ion source 238 is turned on to deposit material layer 215. After layer 215 is deposited, ion beam source 238 is turned off and target 296 is rotated to desired sputtering position 259 to deposit material layer 217. It is well know by those skilled in the art that a time for rotation for target holder 250 is approximately 5 seconds to 10 seconds per target. The time for rotation is essentially wasted since apparatus 203 is idle while target holder 250 rotates.
Turn now to FIG. 2 which illustrates a pie chart 90 for a throughput of ion beam deposition apparatus 203. The throughput measures a number of wafers processed by apparatus 203 per hour. Pie chart 90 represents a percentage of time that apparatus 203 spends performing certain tasks during a specific deposition sequence. For example, apparatus 203 spends approximately 47% of its time depositing material layers, 16% of its time transferring wafers in and out of chamber 210, 12% of its time panning substrate 214, and 1% of its time burning in targets. The latency time of ion beam deposition apparatus 203 is 24%. It will be understood that these values depend on the material stack that is deposited.
The latency time is that when apparatus 203 is sitting idle waiting for a task to finish. As mentioned previously, target holder 250 takes approximately 5 seconds to 10 seconds to rotate from one target (i.e. target 258) to a next target (i.e. target 262) in a deposition sequence. Also, it is well known by those skilled in the art that various wafer cleaning steps and beam setup steps are performed during the deposition sequence. Further, it is well known by those skilled in the art that it is desirable to maximize the percentage of time apparatus 203 spends depositing films, which will increase the throughput. For example, apparatus 203 has a throughput of approximately 4.9 wafers per hour.
Accordingly, it is an object of the present invention to provide a new and improved ion beam deposition apparatus with an improved throughput and improved shielding.
To achieve the objects and advantages specified above and others, a method of depositing a material by sputtering is disclosed. The method comprises the steps of providing a vacuum chamber and providing a substrate positioned in the vacuum chamber. A first target holder capable of holding at least one first target holding member is positioned in the vacuum chamber, said first target holder being rotatable about a first individual axis. A second target holder capable of holding at least one second target holding member is positioned in the vacuum chamber proximate to the first target holder, said second target holder being rotatable about a second individual axis.
A first ion beam source for directing ions at the first target holder is positioned in the vacuum chamber and a second ion beam source for directing ions at the second target holder is positioned in the vacuum chamber and positioned proximate to the first ion beam source. One of the first plurality of target holding members which bears a first desired sputtering target is positioned into a first sputtering position. A desired one of the second plurality of target holding members which bears a second desired sputtering target is positioned into a second sputtering position.
The first ion beam source is used to deposit a portion of the first desired sputtering target onto the substrate to form a first material layer. The first ion beam source is then turned off and the first target holder is rotated to a third desired sputtering target while using the second ion beam source to deposit a portion of the second desired sputtering target onto the first material layer to form a second material layer. Further, the second ion beam source can be turned off while using the first ion beam source to deposit a portion of the third desired sputtering material onto the second material layer to form a third material layer.